[gmx-users] The Cut-off for coulombtype heat up the water system?

[XAvier Periole]

It is simply that if the system is not neutral the Ewald equation are  
not valid anymore. I would guess that the original papers describe it.  
I never looked into it.

On Jun 19, 2009, at 21:02, Florian Dommert <dommert at icp.uni- 
stuttgart.de> wrote:

> * Mark Abraham <Mark.Abraham at anu.edu.au> [2009-06-19 14:55:02 +1000]:
>
>> Chih-Ying Lin wrote:
>>> Hi
>>> People conclude that the heating up is normal by using a plain cut- 
>>> off.
>>> So, how to fix the problem?
>>
>> 0. Do more background reading. :-)
>>
>>> 1. From Berk => use multiple groups.
>>>    => how  ???
>>>    => I have been thinking that it is better to group the molecule
>>>    => such as: protein , non-protein
>>>
>>> 2. change the coulombtype without the coulomb cut-off rcoulomb ?
>>>    => such as PME, PPPM ?
>>>    => what's the suggestion about this?
>>>
>>> 3. Normally, how do people fix this problem?
>>
>> These days, PME will tend to be the easiest to defend in a  
>> publication.  You will have lower heating problems with various  
>> modified forms of  cut-offs and/or longer cut-offs, but then you  
>> have the problem of  justifying the use of the force field, which  
>> was probably parametrized  for some other coulomb scheme.
>
> Hi Mark,
>
> Can you please explain me or provide am reference explain why I am not
> able to use a SPME/PME/PPPM/EwaldSum method for any kind of partial
> charges living in an environment with periodic boundary conditions ?
>
> When I understood the idea of the reaction field correctly, I treat  
> the
> electrostatic forces with a cutoff and relative dielectric  
> permittivity
> != 1. With the mentionend Ewald methods I should be able to reproduce
> exactly the same circumstances like in a reaction-field setup. So at  
> the
> moment I can imagine just one critical point, when using SPME/PME/PPPM
> or an Ewald sum is the big set of parameters that have to adapted in
> order to obtain an appropriate accuracy of the forces. In the reaction
> field method you just have two parameters: the cutoff and epsilon_r.  
> The
> other algorithms require addtionally require the input of an  
> appropriate
> size for used grid in Fourier space and in case of SPME/PME/PPPM  
> also an
> interpolation order. Finally you need to set the splitting paramter
> correctly, otherwise you will obtain unaccurate forces. So there can  
> be
> a very large error introduced, when applying the wrong parameters to  
> the
> Ewald methods. The heat up of the water is also just related to  
> extremly inaccurate
> electrostatic forces, since with PBC an "infinite" system is  
> simulated and just a very small amount of the electrostatic  
> interaction that is of
> long range nature is calculated. Therefore an large error is not  
> unexpected.
> Finally the only restriction of Ewald I see is the requirement of PBC,
> where I can reach any level of accuracy for the electrostatic force
> given by certain charge distribution, don't I ?
>
> However I am always open to enhance my experience and knowledge
> therefore please let me know if understood anything wrong.
>
> Flo
>>
>> Mark
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>
> -- 
> Florian Dommert
> Dipl.-Phys.
>
> Institute for Computational Physics
> University Stuttgart
>
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> 70569 Stuttgart
>
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>
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